Electrically insulating polyproplyene laminate paper and oil-impregnated electric power cable using said laminate paper

ABSTRACT

An insulation for electric cables which comprises a biaxially oriented polypropylene film bonded to an oil-impregnated paper by means of a melt-extruded polyolefin adhesive and the resulting insulated cable are disclosed.

, United States Patent 1191 1111 3,775,549 Nov. 27, 1973 Matsuda et, a1,

[ ELECTRICALLY INSULATING ,POLYPROPLYENE LAMINATE PAPER ANDOIL-IMPREGNATED ELECTRIC PowER CABLE USING SAID LAMINATE P PER 7'5lnventorsz 51111 11 Matsuda; Hidemitsu Kuwabara, both of Shizuoka;Hiroshi Kubo, Osaka; Yoichi Sasajima, Osaka; Kensuke Suzuki, all ofOsaka; Takashi Fukamachi, Tokyo, all of Japan [73] Assignees: SumitomoElectric Industries, Ltd.,

Osaka; Tomoegawa Paper Manufacturing Co., Ltd., Tokyo, Japan [22] Filed:June 23, 1972 [2!] Appl. No.: 265,896

[30] Foreign Application Priority Data 174/110 PM, 120 R, 120 FP,120 SR;

[56] References Cited UNITED STATES PATENTS 3,194,872 7/1965 Garner174/25 R 3,450,968 6/1969 Cox 174/25 R X 3,560,227 2/1971 Eichom et al.161/250 X 3,634,546 l/1972 l-lagemayer et a1 161/250 X 7 3,594,4897/1971 Katz 174/110 PM X 3,370,106 2/1968 Hall, Jr. 161/250 X 3,607,9879/1971 Walton 161/250 X 3,497,574 2/1970 Press 161/250 X 3,542,71711/1970 Lipman.... 161/250 3,380,868 4/1968 Oriskany... 161/252 X3,671,383 6/1972 Sakata eta 161/252 Primary Examiner-Bernard A. GilheanyAssistant Examiner-A. T. Grimley Attorney-Richard C. Sughrue et a1.

[5 7] ABSTRACT An insulation for electric cables which comprises abiaxially oriented polypropylene film bonded to an oilimpregnated paperby means of a melt-extruded polyolefin adhesive and the resultinginsulated cable are disclosed.

12 Claims, 7 Drawing Figures no. m

RATENTEURU ETUTT 3775-549 5200- gag OIL PRESSURE. lKg/ 2 gm CABLE MODEL2 g; [50- LENGTH m m INSULATION E THICKNESS g TAPE WIDTH lgmm c VENTI ALSAMPLE OF ABLE TrODEL CABLE MODELOF INSULATED CELL- OSE PAPER TAPES'NVENT'ON POLARITYANDY v TEMPERATURE $9 @9 80C H6 4 I ES/El L7 L2 v PATENTEUuuv 27 m;

BLT/5549 sum 3 OF 3 IOO cm (s095c PRESSURE DIFFERENCE CABLE LENGTHTEMPERATURE OF OIL IMMERSED PERI0D (0AY) FIG. 5

BACKGROUND OF THE INvENTIoN FIELD OF THE INvENTIoN nate paper.

DESCRIPTION OF THE PRIOR ART In recent years, electric machinery such asoilimpregnated electric cables as a typical example which withstandextra high voltages have been developed, and on the other hand, for thepurpose of reducing the cost, there is a tendency towards the productionof smaller size electric machinery. Therefore, solid insulating materialfor use in suchelectric machinery, especially, electrically insulatingtapes, require superior dielectric characteristics such as dielectricloss tangent (tan 8) or dielectric breakdown. strength against impulsevoltage or AC. voltage, and also superior mechanical strength andoperability.

Attempts have been made to use plastic films having low tan 8 as such anelectrically insulating tape, instead ofthe conventionalelectricallyinsulating paper. The plastic films have very good initialimpulse strength dielectric characteristics, but have the defect thatthey are drastically deteriorated in resistances to repeated applicationof impulse voltage or to AC voltage for prolonged periods'of time. Inaddition, they have the temperature dependent characteristics inherentin plastic films or have large polar effects on impulse voltage. Asfurther disadvantages, the plastic films have poor work ing efficiency,for example in tape winding operations, because of lack of rigidity, andthe packing action inherent in the plastic films makes it difficult todry electric cables or other electric machinery under vacuum which inturn leads to poor flowability of insulating gases-or oils.

On the otherhand, oil-impregnated paper has inferior dielectriccharacteristics or electric breakdown strength compared to the plasticfilms, but is superior in other respects. However, the tan of oil'-impregnated paper is considered to be 0.1 percent at the lowest, andtemperature rise due to tan 8 loss exerts I limitations on theapplication of electric cables or other equipment to extrahigh voltageand on the reduction in size of such electric equipment.

A system has been devised which consists of alternate winding of theabove-described plastic film and oilimpregnated paper in an attempt toutilize the merits of these two materials. However, when this system isso arranged that the plastic film faces the conductor, it is weak topositively polar impulses. In order to avoid this, the electricallyinsulating paper may be first wound facing the conductor and then theplastic film is wound thereon, in the case of transformers orcapacitors. With electric cables, a larger proportion of the plasticfilm faces the conductor side in an oil layer and the defect of weaknessto positively polar impulses is not eliminated. In this alternatelywound system, too,

the plastic film of small rigidity needs to be wound alone, andtherefore, improvement in working efficiency can hardly be hoped for.

SUMMARY OF THE INVENTION The present invention has eliminated the'abovementioned defects, and its first feature is to provide anelectrically insulating polypropylene laminate paper comprising anintegrated assembly of a biaxially stretched polypropylene film and anelectrically insulating paper bonded to each other through the medium ofa molten polyolefin such as polypropylene or an ethylene/propylenecopolymer.

A second feature of this invention is to provide an electric cablewherein a tape of the above-mentioned electrically insulatingpolypropylene laminate paper is wound on an electric conductor to forminsulation layers, and an insulation oil is impregnated therein.

A third feature of this invention is to provide an electric cableimpregnated with an electrically insulating oil by winding theelectrically insulating polypropylene laminate on a conductor while theelectrically insulating paper is being rendered wet, and then drying itto remove moisture, which process is based on the utilization of themoisture absorbing characteristics of the electrically insulating paper.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a sectional view of a single-core OF cable of the presentinvention.

FIGS. 2(a) and 2(b) show the results of an impulse breakdown test on asheet impregnated with an alkylbenzene oil.

FIGS. 3(a) and 3(b) are enlarged sectional views of the insulatinglayer(s) of the present invention.

FIG. 4 shows the result of an impulse breakdown test on an OFpaper-polypropylene sheet impregnated with an alkylbenzene oil.

FIG. 5 shows the variation in the amount of oil which is impregnatedinto different types of cables with immersion time in the oil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS film. We have made an extensiveresearch and develop-.

ment work on the production of an insulating material by combining apolypropylene film with an electrically insulating paper, and found thata polypropylene laminate paper comprising an assembly of a biaxiallyoriented polypropylene film and an electrically insulating paper bondedto each other by a molten polyolefin proves excellent.

It has previously been attempted to use an insulating material producedby laminating polypropylene on an electrically insulating paper by theextrusion process. According to this process, the polypropylene used isan unoriented polypropylene which has inferior properties to biaxiallyoriented polypropylene.

This was experimentally confirmed as follows: Polypropylene was extrudedin the molten state in a thickness of p. on the extrusion process onto a12511. m thick insulation paper for electric cables (to be abbreviatedto OF hereinafter). The resultant laminate paper was designated as (A).A 60y. m thick biaxially I Biaxially oriented polypropylene films arerelatively oriented polypropylene film was superposed on OF 125 low incost, and are superior to polyethylene films in while melt-extrudingpolypropylene in a thickness of heat resistance, oil resistance andelectric breakdown 30p. m onto OF 125 by the extrusion process. Therestrength. Therefore, they are preferred materials for suiting laminatepaper was designated as (B). The A.C. 5 laminate insulations based on aninsulating paper. The breakdown strength, oil resistance and oil flowresisinventors of the present invention studied an insulation F i 9 s P(A) and (B) were measured. material comprising an integrated assembly ofan insu- The results are given in Table 1. lating p p and 3 biaXiallyoriented polypropylene film on the basis of the overall consideration ofthe above- TABLE 1 mentioned results.

Coefficient We then studied a method of incorporating an elec- AC Oil ofinherent Breakdown Resistance on How trlcal insulating paper with abiaxially oriented polypro- Strength (Swelling Resistance pylene film.As a result, we have experimentally found (a) ffig sz Ram I that aninsulation material consisting of an electrically poi'ypm'pylgne 113-0Insulating paper/an unorlented polyolefin/a biaxially u 108-7 1 113x1013oriented polypropylene film which was obtained by g bonding the firstand third components with the second nm/biaxially component in themolten state by the extrusion process ggf gl g g 6O w 7 v V M H is byfar superior. Examples of the polyolefin as an adum 121.6 3 3.0 10hesive are polypropylene and an ethylene/propylene V copolymer. Theexperimental results will be described l rifi b le l", the x'ct'retkaawa"Stran ers "a time below.

measured at 60 Hz with reference to OF oil- It is known that a biaxiallyoriented polypropylene impregnated flat plate sample. The oil resistanceis a film has poor adhesiveness. On the other hand, it is necratio(swelling ratio) of the thickness of the sample beessary to use anadhesive which does not adversely affore impregnation of OF oil to thatof the sample which feet the dielectric characteristics of theinsulation. In has been immersed for days at 80 C in OF oil and theexperiments, laminate papers prepared by various completely swollen withOF oil. FIG. 5 shows that the known methods, and integrated assembliesof an elecquantity of oil exhausted from cable model for 30 mintricallyinsulating paper anda biaxially oriented polyutes under constantpressure difference is plotted 30 propylene film bonded to each otherbyan extrusionagainst immersed period at elevated temperature. The moldedmolten polypropylene or ethylene/propylene coefficient of inherent oilflow resistance in Table l copolymer adhesive were compared with eachother in means the value calculated from the value at the staurespect ofbond strength, oil resistance; tan 8 and imrated point in FIG. 5. Thecoefficient of inherent oil pulse strength. The results are showninTable 2. As flow resistance is the oil resistance value at completesamples, a 60 mp thick biaxially stretched polypropylequilibrium of amodel cable produced by using sample ene film and a 35 pm thickinsulating paper for electric (A) or (B), which value has been correctedwith refercables were used.

W. TAELE 2 W A Peeling Impulse strength strenglth (g./l5 Tan, (K Samplesmm.) Oil resistance percent mm.)

(A) Integrated using molten polypropylene as an adhesive by theextrusion No part peeled off 0. 030 327 recess. (B Integrated usingchlorinated polypropylene as an adhesive 0.5 Completely separated intothe 121- 0.775 325 axially stretched polypropylene film and theinsulation paper. (C) Integrated using chlorinated rubber 8 .-...d0.1.013 302 (D) Integrated using polyurethane as an adhesive 4 do O. 142310 (E) The biaxially oriented polypropylene was heatbonded to theinsulating 45 No part. peeled ofi; but the surface 0.035 280 paper.became roughened.

(F) Integrated using a molten ethylene/propylene copolymer as anadhesive 138 No part peeled off 0. 031 318 by the extrusion process.

e'n''io the oil pressurefnieasurement length, viscosity The pea strength is a v alue of ilf'stfrengtlfof a 15 of OF oil, insulationthickness and conductor diameter. mm wide sample measured by a tensiletester. The oil Larger values show that the inherent oil flowresistancev resistance is evaluated by observing the shape of the of thematerial is large. It is seen from the results shown sample after it hasbeen immersed for 5 days at 80 C in Table 1 that the sample (B)consisting of the electriin an alkylbenzene. The tan 8 is a valuemeasured at cal insulation paper and the biaxially oriented polypro- Hzat C while the sample contains the alkylbenzene pylene film is superiorin AC breakdown strength and impregnated therein. The impulse strengthis a value oil resistance. A greater difference is the oil flow resis-60 measured with respect 'to a flat plate sample impregtance. Experienceof the inventors of this invention innated with the alkylbenzene same asin the case of the dicates that if a cable has a coefficient of inherentoil impulse strength, with the insulating paper being diflow resistanceof about 6.2 X 10 cm or less, it can rected to the side of the positivepole.

be used without any disadvantage against transient As shown in Table 2,samples (A) and (P), which fluctuations in oil pressure which areinherent to ca- 6 were produced by melting polypropylene or an bles:FI'OmIhIS VIeWPOmI, the Sample COHSlStlHg 0f ethylene/propylenecopolymer by the extrusion process the lnsulatlng paper nd theunorlented p lypr py to make them filmy, and bonding a biaxiallyoriented poses a problem 111 P acti efilm of polypropylene to anelectrical insulating paper using said molten polypropyleneorethylene/propylene copolymer as an adhesive, were best balanced inrespect of bondystrength, oil resistance, tan and impulse strength. Thebiaxially oriented polypropylene film had "been Stretched Ito 48X"(longitudinal stretch x 3 transverse stretch)..- 1 i lna laminateinsulatingpaper obtained by bonding a latingpaper by a molten polyolefinin the abovementionedmanner, the low rigidity of the biaxially ori-,insu'lating paper faces the conductor, the polypropylene film surfacedoes not face the conductor in an oil layer. Therefore, such aninsulated cable does not produce'a polar effect which isseen in theconventional plastic tape-wound cable which hasweak resistance topositively polar impulses, and moreover proves far superior tooil-impregnated paper in respect of electric breakdown strength anddielectricloss.

' Changes in electrical-and physical characteristics ac cording to thethickness of the polyolefin asan adhesive were examined with respect topolypropylene (unoriented polypropylene). Generally, an unorientedpolypropylene filmhas inferior electric characteristics and oilresistance as compared to a biaxially oriented polypropylenefl film, andtherefore, it is expected that the molten polypropylene should better bemade into a film which is as thin' as can retain thebond strength of themolten polypropylene. The inventorsof the present invention confirmedthis by' basic experiments. Table 3 shows how the impulse strength andoil resis;

tance (swelling property) of a laminate paper obtained by bonding a 40pm'thickbiaxially oriented polypropylene film to a 70 pm thickinsulating paper for electric cables using molten polypropylene adhesivein. accordance'with the extrusion process change with the thickness ofthe polypropylene film adhesive. The test was conducted by immersing thesample in an alkylbenzene and heating it at 80 C for 20 days.

TABLE 3 Thickness of molten Impulse Strength Swelling Ratiopolypropylene (urn) (KV/mm) 270 5.0 250 6.2 30 233 7.2 50 201 8.8 70 177Y 10.0

* The ratio of the thickness of the sample before kmpregnation to thatof the sample which was heated at 80 C for 20 days in the alkylbenzene.

- It is seen therefore that with increasing thickness of thepolypropylene film adhesive,the impulse strength decreases, and theswelling of the sample by oil be- 2 comes greater. Hence, the thicknessof the polypropylene film adhesive should preferably be as thin aspossible. This swelling can be reduced by the compressioncharacteristics of the biaxially oriented polypropylene film andelectrically insulating paper. However, if a thin electricallyinsulating paper having low strength is l j,;,. i 3,775,549

biaxially oriented polypropylene to anelectrically insuused, it maypossibly be cut by the swelling force of the polypropylene in thethickness direction, In view of this also, it is preferred that thethickness of the polypropylene film adhesive should be as thin aspossible. To confirm this, we have conducted the following experiment.

The samples shown in Table 3 were cut into tapes with a width of 10 mm,with the applicationof a tension of 1 Kg, each of the samples was woundon a glass tube having a diameter of 12.5 mm in a thickness of about 5mm, followed by immersion in an alkylbenzene. The

sample was then heated at C, and then taken out after a lapse of 30days. The-sample was unwound and it was found that the sample in whichthe thickness of the polypropylene film adhesive was 70 p. m torn offpartially at the part of the insulating paper. This shows that owing tothe swelling of the polypropylene, the paperwas cut. The results of thisexperiment showed that the thickness of the polypropylene film adhesiveshould be about three-fourths or less of the thickness of the insulatingpaper to be used. In order to prevent the tear of the paper by theswelling of polypropylene,

thick paper of high strength may be used. But if the proportion of theinsulating paper in anassembly of the insulating paper, moltenpolypropylene and a bi-axially oriented polypropylene film becomeslarge, the advantage of using the biaxially oriented polypropylene filmhaving excellent electrical properties is reduced. Accordingly, theinsulating paper to be used should also be as thin as possible.

The biaxially oriented polypropylene film is available in a wide varietyof thicknesses. Those having large thickness (about p. m or more) causea reduction in electric breakdown strength, and therefore, in thelaminated paper of this invention, the thickness of the biaxiallyoriented polypropylene film should preferably be not more than about 100p. m.

Confirmatory experiments relating to the influences of the thickness ofthe insulation paper to impulse strength will be described below. v

Samples were prepared by bonding a 60 p. m biaxially orientedpolypropylene film to an insulating paper of various thicknesses using15 p. m thick polypropylene as adhesive in accordance with the extrusionprocess. Each of these samples was immersed in a cable oil, and changesof the impulse strength of the sample according to the thickness of theinsulating paper were examined. The results are given in Table 4.

TABLE 4 Thickness of the insulating paper impulse Strength (M (K /mm) 40330 70 266 222 184 200 x40 strength of this sample were measured. Theresults are ow in TilPlFi I defect of low impulse strength possessed byapolypropylene fiber-'basedelectrically insulating paper can beeliminated drastically by incorporating it with the biaxiallyorientedpolypropylene film described above. Furthermor'e, by using analkylbenzene which has low dielectric constant (e), the dielectricconstant (e) of the entire composite can be further reduced, and the tan8 also decreases.

In the following, oil-impregnated electric power cables using thepolypropylene laminate paper of this invention will be described.

FIG. 1 shows a sectional view of a single-core OF cable in accordancewith this invention. The reference numeral 1 represents a copper oraluminum conductor; 3"! er al..ssmircq d s ei stall; an t rnasemi-conductor layer; 3, an insulation layer composed of a tape of apolypropylene laminate paper of this invention consisting of a biaxiallyoriented polypropylene film and an electrically insulating paper andbeing impregnated with an insulating oil, forexample OF-oil,dedecylbenzen e tridecylbenzene mono-dialkylated naphthalene; 4, a metalsheath; and 5, an anti-corrosive layer. t

Of late, superhigh voltage cables have been developed. In order toprovide at low cost an extra-high voltage cablehaving a'thin insulationlayer and a decreased outer diameter which can be wound ona small-sizeddrum, the alternate current working stress should be increased to ashigh as 20 to 30 KV/mm from 7 to KV/mmwhich is in current use.Therefore, the impulse, AC strength and tan 8 (dielectric loss tangent),which pose a problem in setting the working stress, should haveexcellent stability over prolonged periods of time. A number of attemptshave previously been made to employ plastic films, such as polyethyleneor polycarbonate films, having low dielectric constant (e) and low tan 8in extra-high voltage cables, as already mentioned above. These plasticfilms have good initial voltage resistance characteristics but have thedefect of being considerably deteriorated in breakdown characteristicsagainst repeated application of impulse or AC voltage for prolongedperiods of time, and also have the disadvantage that they havetemperature'dependent characteristics, a property inherent in plasticsand exert a large polar effect. Furthermore, creases may occur at thetime of manufacture of the cables or building a transmission systemusing the cables, or because of packing action between the plasticfilms, there are problems of poor vacuum formation, poor impregnation ofoil, and bad oil resistance. These difficulties set limitations on thepractical use of the plastic films for cable insulation. On the otherhand, oil-impregnated paper has been in wide use as an insulation layerof selfcontained or type-filled oil-incorporated electric cables, ofcablesof 60'KV to 500'KV because of its superiority in variousproperties other than dielectric constant and dielectric loss tangent.However, the dielectric constant of such oil-impregnated'paper islimited to 3.4-3.7, and its dielectric loss tangent is 0.1 percent atthe lowest. Therefore, limitations are imposed by temperature increasesowing to tan 8 loss, and it has been considered difficult to buildcables of l the order of 1,000 KV even if forced cooling is applied.

In view of the above, the cable of this invention is so designed thatthe ratio of e of an oil-impregnated tape layer to that of an insulationoil'(e ,/e, is limitedto 1-1.5, whereby stress on oil has beendrastically reduced as compared with the conventional OF cables, and thevoltage resistance of the entire cable has been improved.

FIGS. 2-(a) and 2-(b) show theresults of an impulse breakdown test on asheet impregnated with an alkylbenzene oil. FIG. 2-(a) shows that whenan electrically insulating paper having high air-impermeability isintegrated with a biaxially oriented polypropylene film, an increase involtage resistance can be obtained. With an air-impermeability of 200Gurley seconds, the voltage strength is considerably decreased, and forpractical purposes, this is a minimum allowable value. Paper having an'air-impermeability lower than this value is unsuitable. FIG. Z-(b) showsthat when the insulating paper is provided on an oil layer surfacefacing the conductor, there is no polar effect. of impulse strength,

which is inherent in plastics- These facts indicatethat theair-impermeability of the electrically insulating paper should be atleast 200 Gurley seconds, and that the winding of the electricallyinsulating paper to face the conductor side is effective. Since paperwith high air-impermeability is expected to have a great effect oftrapping ions or electrons generated by a strong electric field, it isconsidered to contribute to an improvement in voltage strength. When inan OF cable utilizing a plastic film and an oil layer is provided on theside of a conductor, and the plastic surface is in contact with the oillayer, application of impulse of positive polarity to the conductor sidecauses a decrease in the intensity of the impulse owing to the collisionof positive. ions, etc. In other instances (for example, when an impulseof negative polarity is applied, or a cushioning material such as aninsulating paper is interposed between the oil layer and the plasticsurface), a decrease in voltage strength does not occur even byapplication of impulses of positive polarity.

For practical purposes, the insulation layer of a cable may be built byalternately winding a biaxially oriented polypropylene film and anelectrically insulating paper, but since the polypropylene surface morefrequently faces the conductor side in the oil layer, the voltagestrength decreases for the abovementioned reason and the polar effect ofthe impulse becomes greater. This method is therefore not so preferred.

FIG. 3-(a) and '3-(b) are enlarged sectional views of the insulatinglayer produced according to this invention. The reference numeral 1represents a' copper or an impulse-breakdown strength test. Theresultsare shown in FIG. 4. a I v It is seen fromthis figure thattheimpuls e strength of this cable improved about 50 percent, over theconventional OFmodel cable. It can be concluded-from these resultsthatthe working. stress near the conductor can be increased to m 30KV/mm, and there can be produced cables of the order of 1,000KV having adrastically reduced insulation thickness and being capable of beingwound up on a drum.

The electric cables in which the polypropylene laminate paper of thisinvention is wound possess the characteristics of boththe plastic andinsulating paper, and are free from creases which pose problems at thetime of building a transmission system using the cables and haveimproved oil impregnating properties as well as improved electricalproperties. V

In other words, by suitably controlling the thickness of biaxiallyoriented polypropylene to be bonded to a fiber-based paper,dielectricconstant (6) can be easily controlled within 2.2-3.4, and the electricfield can be relaxed by the difference in e of the insulation layer.

Improvement in voltage strength can be achieved since the ratioof thedielectric constant between the insulation tape layer and theinsulationoil (gle approaches l and the voltage can be borne by thepolypropylene laminate paper having highvoltage strength. Furthermore,the dielectric loss tangent of the cable can be maintained at 0.05percent or less, which is considered to be difficult with anoil-impregnated paper,

owing'to the low'tan '6 characteristic of biaxially orientedpolypropylene...

Furthermore, there can be produced electrically stable cables 'orreduced temperature effect or polar effect on impulse. Because of theseadvantages, the working stress increases, and the insulation thicknesscan be made smaller, which in turn leads to a drastic curtailment ofcost for production of cables of the order of 60 to 500 KV. It is alsopossible to build cables of the order of 1,000 KV which can be wound ona drum unlike the conventional ones. From the process viewpoint, thedrying step can be shortened because polyfilm and being as small aspossible without substantially sacrificing the bond strength exhibitedbetween said electrically insulating paper and said polypropylene 4. Thelaminate of claim 3 wherein the thickness of g i said biaxially orientedpolypropylene film is not more than 100 um.

' '5. The laminate of claim 3 wherein thethickness of said polyolefinadhesive is not more than 0.75 times the thickness of said electricallyinsulating paper.

.6. An oil-impregnated electric power cable comprising an electricalconductor and insulation surrounding said conductor, at least a portionof said insulation comprising an electrically insulatingpolypropylenepaper laminate consisting of an integrated assemblyselected from the group consisting of polypropylene and a propylenecopolymer containing a major proportion of propylene units, thethickness of said adhesive being less than the thicknesses of each ofsaid paper and said film and being as small as possible withoutsubstantially sacrificing the bond strength exhibited between saidelectrically insulating paper and said polypropylene film, saidelectrically insulating polypropylene-paper laminate having anelectrically insulating oil impregnated therein.

7. The electric power cable of claim 14 wherein said electricallyinsulating polypropylene-paper laminate is wound around saidconductorwith the paper side of said laminate facing the conductor and whereinsaid electrically insulating paper has an air-impermeability of at least200 Gurley seconds.

propylene is non-hygroscopic. The cables of this invenless than thethicknessof each of said paper and said 8. The electric power cable ofclaim 7 wherein the thicknessof said biaxially oriented polypropylenefilm is not more than um and wherein the thickness of said polyolefinadhesive is not more than 0.75 times the thickness of said electricallyinsulating paper.

9. The electric power cable of claim 7 wherein said polyolefin adhesivecomprises an unoriented polyolefin.

10. The electric power cable of claim 7 wherein the ratio of thedielectric constant (e) of said electrically insulatingpolypropylene-paper laminate to the dielectric constant (e) of saidelectrically insulating oil varies from 1 to 1.5.

11. The electric power cable of claim 7 wherein the dielectric losstangent thereof is not more than 0.5 percent.

12. The process for producing the electric power cable of claim 15comprising:

l. contacting said paper with water wherein said polypropylene-paperlaminate absorbs moisture;

2. winding the moisture-containing polypropylene paper laminate aroundsaid conductor while said laminate is wet; 3. drying to remove themoisture; and 4. impregnating said laminate with said oil.

2. The laminate of claim 1 wherein said polyolefin adhesive is anunoriented polyolefin.
 2. winding the moisture-containingpolypropylene-paper laminate around said conductor while said laminateis wet;
 3. drying to remove the moisture; and
 3. The laminate of claim 2wherein the air-impermeability of said electrically insulating paper isat least 200 Gurley seconds.
 4. The laminate of claim 3 wherein thethickness of said biaxially oriented polypropylene film is not more than100 Mu m.
 4. impregnating said laminate with said oil.
 5. The laminateof claim 3 wherein the thickness of said polyolefin adhesive is not morethan 0.75 times the thickness of said electrically insulating paper. 6.An oil-impregnated electric power cable comprising an electricalconductor and insulation surrounding said conductor, at least a portionof said insulation comprising an electrically insulatingpolypropylene-paper laminate consisting of an integrated assemblyselected from the group consisting of polypropylene and a propylenecopolymer containing a major proportion of propylene units, thethickness of said adhesive being less than the thicknesses of each ofsaid paper and said film and being as small as possible withoutsubstantially sacrificing the bond strength exhibited between saidelectrically insulating paper and said polypropylene film, saidelectrically insulating polypropylene-paper laminate having anelectrically insulAting oil impregnated therein.
 7. The electric powercable of claim 14 wherein said electrically insulatingpolypropylene-paper laminate is wound around said conductor with thepaper side of said laminate facing the conductor and wherein saidelectrically insulating paper has an air-impermeability of at least 200Gurley seconds.
 8. The electric power cable of claim 7 wherein thethickness of said biaxially oriented polypropylene film is not more than100 Mu m and wherein the thickness of said polyolefin adhesive is notmore than 0.75 times the thickness of said electrically insulatingpaper.
 9. The electric power cable of claim 7 wherein said polyolefinadhesive comprises an unoriented polyolefin.
 10. The electric powercable of claim 7 wherein the ratio of the dielectric constant ( epsilon) of said electrically insulating polypropylene-paper laminate to thedielectric constant ( epsilon ) of said electrically insulating oilvaries from 1 to 1.5.
 11. The electric power cable of claim 7 whereinthe dielectric loss tangent thereof is not more than 0.5 percent. 12.The process for producing the electric power cable of claim 15comprising: